1. Field of the Invention
The present invention relates to traction modifiers of the positive drive type and in particular to an improved structure for dampening shutter in positive drive devices.
2. Description of the Prior Art
Traction modifying devices are well known in the prior art and are usually categorized as limited slip differentials, such as are illustrated in U.S. Pat. No. 3,611,833, locking differentials such as illustrated in U.S. Pat. Nos. 2,978,929 and 3,831,462 or positive drives. Traction modifying devices have become popular for use in the drive trains of vehicles between the vehicle transmission and the driven wheels. The primary function of a traction modifier is to facilitate driving on slippery surfaces and off-road conditions. Traction modifiers of the positive drive type are well known in the prior art and examples thereof are illustrated in U.S. Pat. Nos. 2,060,558; 2,179,923; 2,720,796 and 4,400,996, and in published UK patent application GB No. 2,119,040A, the disclosures which are all hereby incorporated by reference.
In general, a positive drive device includes an input member adapted to be driven by the input driving torque transmitted from the power source and first and second output members adapted to drive the vehicle wheels. First and second clutch means are operably associated with the first and second output members, respectively, and each of the clutch means has a disengaged mode and engaged mode. In the disengaged mode of the clutch means, its respective output member rotates relative to the input member, while in the engaged mode, the output member is maintained in the predetermined rotational relationship with the input member. First and second cam means are operably associated with the first and second clutch means, respectively, to move the respective clutch means from the disengaged mode to the engaged mode in response to movement of the cam means from a neutral position to an actuated position.
While the positive drive described herein is utilized in a drive axle, positive drive devices are also well suited for use in power dividers between a series of tandem axles and/or in transfer cases between driven front and rear axle assemblies.
During substantially straight driving movement of a vehicle employing a positive drive, engine power is transmitted approximately equally to the driven wheels, which rotate at the same speed. During a potential spin-out condition, engine power is transmitted to the driven wheels in proportion to their instantaneous traction capability, whereby the wheels are still driven in the same speed.
When the vehicle employing a positive drive turns a tight corner, power is transmitted only to the slower moving (i.e., the inside) wheel, while the faster moving wheel is permitted to substantially free-wheel, relative to the input. When the vehicle is making a gradual turn, the positive drive drives both wheels at substantially the same speed and transmits slightly more power towards the inside wheel (which slips slightly) than to the outside wheel. Thus, a positive drive performs in a manner similar to an open (i.e. conventional) differential during operating conditions which would make an open differential desireable, and performs in a manner similar to a rigid axle when operating conditions would make a rigid axle desireable.
It is an important feature of a positive drive that the device includes two clutches, preferably friction clutches, either of which may be engaged independently of the other and that engagement of the clutches is the means of torque transmission in the normal drive mode, and when there is a speed differential between the two outputs, only the clutch transmitting torque to the slower turning wheel is fully engaged. It is also known, see copending U.S. patent application Ser. No. 538,030, filed Sept. 30, 1983 and assigned to the same assignee as the present invention, that under certain conditions the clutch normally supplying torque to the faster rotating output should be applied with at least a minimum torque transmitting capacity for dampening purposes to minimize or eliminate so called "shutter" of the positive drive device.
Typically, the first and second clutches are applied by means of first and second clutch cams, respectively, which are axially moved to a clutch engaged position by means of a drive cam which interacts with the clutch cams by means of interacting ramp surfaces, balls provided in ramped cavities, or the like. As torque must be provided to the output member in both the forward and reverse modes of operation of the vehicle, each of the clutch cams will have first rotational position relative to the drive cam wherein the clutch cam is not axially displaced into a clutch engaging position and two positions corresponding to rotational displacement in both directions from the first position wherein the clutch cams will be axially displaced to a clutch engaging position. Accordingly, in operating conditions wherein it is desired to maintain one of the clutches in a disengaged condition, it is important to maintain the clutch cam associated therewith in the first position relative to the drive cam thereof. For this purposed, it is known to utilize interacting teeth or the like, usually referred to as "balking means", to limit the relative rotational freedom between the two clutch cams.
While the prior art traction modifiers of the positive drive type, especially those wherein the clutch driving the faster moving output member is applied with a predetermined torque capacity to minimize the problem of shutter, are highly functionally desireable, the prior art designs have not been totally satisfactory as nonstandard differential housings were required, the components of the prior art devices have been rather complicated and expensive to manufacture and/or assemble, and/or have utilized means to limit relative rotational movement between the two drive cams which are relatively complicated and expensive to produce and/or utilize shutter dampening structure which provides a substantially constant shutter dampening torque connection between the driving cam and the nondriven output which is inadequate for dampening high energy shutter or creates an undesireably high constant drag on the nondriven output.